全文获取类型
收费全文 | 21358篇 |
免费 | 1673篇 |
国内免费 | 1780篇 |
出版年
2024年 | 31篇 |
2023年 | 304篇 |
2022年 | 421篇 |
2021年 | 729篇 |
2020年 | 662篇 |
2019年 | 979篇 |
2018年 | 895篇 |
2017年 | 559篇 |
2016年 | 614篇 |
2015年 | 794篇 |
2014年 | 1442篇 |
2013年 | 1562篇 |
2012年 | 1032篇 |
2011年 | 1391篇 |
2010年 | 1013篇 |
2009年 | 1062篇 |
2008年 | 1079篇 |
2007年 | 1229篇 |
2006年 | 1072篇 |
2005年 | 940篇 |
2004年 | 777篇 |
2003年 | 662篇 |
2002年 | 653篇 |
2001年 | 422篇 |
2000年 | 408篇 |
1999年 | 359篇 |
1998年 | 422篇 |
1997年 | 322篇 |
1996年 | 283篇 |
1995年 | 335篇 |
1994年 | 248篇 |
1993年 | 242篇 |
1992年 | 215篇 |
1991年 | 197篇 |
1990年 | 171篇 |
1989年 | 131篇 |
1988年 | 124篇 |
1987年 | 112篇 |
1986年 | 77篇 |
1985年 | 122篇 |
1984年 | 166篇 |
1983年 | 123篇 |
1982年 | 137篇 |
1981年 | 68篇 |
1980年 | 54篇 |
1979年 | 59篇 |
1978年 | 41篇 |
1977年 | 20篇 |
1976年 | 14篇 |
1974年 | 11篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
2.
F. Kato T. Hino A. Nakaji M. Tanaka Y. Koyama 《Molecular & general genetics : MGG》1995,247(3):387-390
In many species of actinomycetes, carotenogenesis can be photoinduced. The capacity to respond to photoinduction is, however unstable and, in various strains of Streptomyces, is lost at a relatively high frequency. In Streptomyces setonii ISP5395, which normally produces no carotenoids, carotenoid-producing mutants can be obtained following protoplast regeneration. We report here the characterization of a gene, crtS, which was isolated from one such mutant and can confer on wild-type S. setonii ISP5395 cells the capacity to synthesize carotenoids. Sequence analysis of crtS reveals an open reading frame, which shows homology to genes that encode alternative sigma factors in Bacillus subtilis. We propose that crtS encodes a sigma factor which is necessary for the expression of a cryptic gene(s) for carotenoid biosynthesis in S. setonii ISP5395. 相似文献
3.
Concentration factor and biological half-life of 54Mn were determined in three species representing an ecologically and economically important food chain. Green algae (Chlorella spp.), Daphnia magna and yellow perch (Perca flavescens) were exposed to 54Mn in water and assayed for 54Mn uptake. Steady state concentration factors computed from the laboratory data for algae, Daphnia and perch were 4230, 17 000 and 11, respectively. Respective biological half-lives were 1.6, 1.2 and 8.3 days. 相似文献
4.
K.S. Misono R.T. Grammer H. Fukumi T. Inagami 《Biochemical and biophysical research communications》1984,123(2):444-451
Four peptides possessing both natriuretic activity and smooth muscle relaxant activity were isolated from rat atrium and their amino acid sequences determined. The four peptides designated ANF-I, ANF-II, ANF-III and ANF-IV containing 35, 31, 30 and 25 amino acid residues, respectively, were obtained in a molar ratio of 4:60:20:16. The predominant species ANF-II, which may represent the native form of ANF, has the following sequence: (H2N)-G-P-R-S-L-R-R-S-S-C-F-G-G-R-I-D-R-I-G-A-Q-S-G-L-G-C-N-S-F-R-Y-(COOH) in which Cys-10 and Cys-26 are disulfide linked. Cleavage of the aspartyl linkage at position 16 by staphylococcal protease caused complete inactivation, indicating that the ring conformation is essential. The dose-response relationships determined for the four pepdides in relaxing norepinephrine-induced contraction of rabbit thoracic aorta showed half-maximum relaxation at concentrations ranging from 1.5 × 10?9 to 2.5 × 10?9 M. Comparable dose-response relationships were observed in relaxation of carbacol-induced contraction of chick rectum strips as tested with ANF-II and ANF-IV. 相似文献
5.
Eggs of the elm leaf beetle Xanthogaleruca luteola are often heavily attacked by the chalcidoid wasp Oomyzus gallerucae. We studied the chemical signals mediating interactions between the egg parasitoid, its host, and the plant Ulmus campestris. Olfactometer bioassays with O. gallerucae showed that volatiles of the host-plant complex attract the parasitoid. In order to determine the source of attractive volatiles
within this host-plant-complex, we tested separately the effect of odours of eggs, gravid elm leaf beetle females, faeces
of the beetles and elm twigs (with undamaged leaves and leaves damaged either mechanically or by feeding of the beetles).
Odours of faeces of the elm leaf beetle were attractive, whereas neither volatiles from eggs nor from gravid females acted
as attractants. Volatiles from undamaged or damaged plants did not elicit a positive reaction in O. gallerucae, whereas volatiles from feeding-damaged plants onto which host eggs had been deposited were attractive. This latter result
suggests that it is not feeding but deposition of host eggs onto elm leaves that induces the production of plant volatiles
attractive to the egg parasitoid. Investigations of the search patterns of O. gallerucae within the habitat by laboratory bioassays revealed that the egg parasitoid encounters host eggs by chance. Contact kairomones
from faeces were demonstrated to be important in microhabitat acceptance, while contact kairomones isolated from the host
eggs are relevant for host recognition.
Received: 12 February 1997 / Accepted: 29 April 1997 相似文献
6.
Selective increase in cytokeratin synthesis in cultured rat hepatocytes in response to hormonal stimulation 总被引:3,自引:0,他引:3
I Leroux-Nicollet M Noel H Baribault R Goyette N Marceau 《Biochemical and biophysical research communications》1983,114(2):556-563
Addition of a combination of insulin, dexamethasone and EGF at seeding time to cultured rat hepatocytes in serum-free medium caused a selective increase in the biosynthesis of particular cytokeratin components. This increase was prominent during the first day in culture. No significant increases were detected in the absence of hormones or in the presence of either hormones added alone or in pairs, except in the case of insulin plus dexamethasone, which yielded an effect close to that obtained with the three factors. Interestingly, the latter condition also maintained a high level of albumin production over a 6-day period in culture. 相似文献
7.
Antonis E. Koromilas 《Biochimica et Biophysica Acta (BBA)/General Subjects》2019,1863(3):644-649
Cells employ pro-survival and pro-adaptive pathways to cope with different forms of environmental stress. When stress is excessive, and the damage caused by it is unsustainable, cells engage pro-death pathways, which are in place to protect the host from the deleterious effects of harmed cells. Two important pathways that determine the balance between survival and death of stressed cells are the integrated stress response (ISR) and the mammalian target of rapamycin (mTOR), both of which converge at the level of mRNA translation. The two pathways have established avenues of communication to control their activity and determine the fate of stressed cells in a context-dependent manner. The functional interplay between the ISR and mTOR may have significant ramifications in the development and treatment of human diseases such as diabetes, neurodegeneration and cancer. 相似文献
8.
B. E. Silkstone 《Oecologia》1987,74(1):149-152
Summary Low levels of artificial damage were imposed on the leaves of Betla pubescens and B. pendula in early summer 1985. Sixteen tress were used and the damage was distributed throughout the canopy in two ways. It was either randomly dispersed on the tree, or restricted to a localised region of the canopy. Up to 250 leaves/tree were damaged and the position of control leaves was marked as appropriate. At the end of the summer the experimental and control leaves were collected for analysis. Subsequent patterns of grazing by naturally-occurring herbivores were affected by the previous damage to the foliage. Fewer artificially-damaged leaves received subsequent insect grazing damage. The frequecy distribution of insect damage to previously-damaged leaves was significantly different from that of the control leaves, and less leaf tissue was removed from those experimental leaves which did receive subsequent insect attack. The evidence presented suggests that wound-induced changes in leaf palatability do occur in B. pubescens and B. pendula in the field and that a major role is to disperse the damage throughout the canopy. 相似文献
9.
Ayenew Ejigou 《Biometrical journal. Biometrische Zeitschrift》1987,29(8):981-991
The odds ratio is known to closely approximate the relative risk when the disease is rare. Logistic regression models are often used to estimate such odds ratios, but here a different model is used which avoids the assumptions implicit in logistic modelling; it also has the advantage of providing a test of homogeneity for odds rat os in situations where the logistic model cannot. 相似文献
10.
The ECF™ lateral flow assay test is marketed to detect non-pregnancy in mares. The objectives of the present study were to determine the accuracy of the ECF test, the accuracy of the electronic reader accompanying the ECF test, and agreement between two human readers and the electronic reader. Serum samples were collected from anestrus, cycling but not inseminated, and inseminated mares, and were evaluated with the ECF™ test (EDP Biotech Company, Knoxville, TN, USA) at The Ohio State University and at the EDP Biotech Laboratory. Specificity ranged from 0.07 to 0.16, the negative predictive value ranged from 0.15 to 0.33, and accuracy ranged from 0.43 to 0.52. The electronic reader did not add improve the accuracy or predictive values of the test. Based on the electronic reader, 80.0% of the serum samples collected from the anestrus mares were false positives; Readers 1 and 2 had 60.0 and 33.3% false positives, respectively. For samples collected during the estrous cycle, 83.9% were false positives by the electronic reader, whereas Readers 1 and 2 had 43.7 and 26.4% false positives. We concluded that, regardless of whether the test strips were evaluated by a human or electronic reader, this assay was not accurate for determination of the non-pregnant mare. 相似文献